In recent years, a natural energy power generation device for generating power from wind, solar power, geothermal heat, or the like has attracted attention and been put to practical use. The natural energy power generation device is a clean power generation device that does not use any limited resource such as oil but uses an energy resource present in unlimited quantity in nature, and can suppress carbon dioxide emissions. Hence, the natural energy power generation device has been increasingly introduced by companies, governments, and the like, for prevention of global warming.
However, since energy coming from nature varies from moment to moment, output fluctuations are unavoidable in the natural energy power generation device. This poses an obstacle to widespread use of the natural energy power generation device. To remove this obstacle, in the case of employing the natural energy power generation device, it is preferable to build an interconnected (power generation) system that combines the natural energy power generation device with an electric power storage-compensation device having a plurality of sodium-sulfur batteries (secondary batteries) as a main component.
A sodium-sulfur battery has features such as a high energy density, an ability to produce high output in a short time, and an excellent high-speed response. Accordingly, when a bidirectional converter for controlling charge and discharge is added, the sodium-sulfur battery offers an advantage that output fluctuations of the natural energy power generation device which can occur on the order of several hundred milliseconds to several seconds can be compensated. Therefore, the interconnected system in which the natural energy power generation device is combined with the electric power storage-compensation device having the plurality of sodium-sulfur batteries as the component is a desirable power generation system.
However, since power generated by the natural energy power generation device instantaneously fluctuates in accordance with its energy source, input or output of power is frequently repeated in the electric power storage-compensation device. This means the sodium-sulfur battery forming the electric power storage-compensation device continuously repeats charge or discharge. As a result, a battery discharge capacity of the sodium-sulfur battery cannot be managed accurately, leading to a problem that the sodium-sulfur battery suddenly reaches a charge end and becomes unable to continue charge or suddenly reaches a discharge end and becomes unable to continue discharge, and stops in the middle of compensating for output fluctuations of the natural energy power generation device.
In view of this, it has been proposed to solve the above-mentioned problem, by detecting a charge end neighborhood or a discharge end neighborhood and changing an output power control target value of a bidirectional converter so that the battery discharge capacity approaches a median value of a rated battery capacity (e.g., see Patent Document 1).